Auxetics are materials that have a negative Poisson ratio — when they are stretched, they get fatter instead of thinner. This is possible because of their underlying structure. One might imagine a foam made out of millions of tiny bow-tie shaped cells, connected to one another. If someone pulls on the sides of the material, the bow ties expand into squares, expanding on the transverse plane as well as the plane parallel to the stretching action. This phenomenon is caused by the macrostructure or microstructure of the material and not the chemical composition of the material itself, so many common materials can be put into auxetic arrangements, although materials that are flexible and stretchy work best.
The concept of materials with a negative Poisson ratio was first published in Science magazine in 1987 by Rod Lakes of the University of Iowa, who has been a leader in the nascent field. The term "auxetic" was not used to refer to these materials until about 1991. It was derived from the Greek word auxetikos, which means "that which tends to increase."
No Natural Examples
Auxetic materials are not natural, and no known biological examples exist. The first auxetics were foams with specifically engineered microstructures. Depending on the size of the air gaps in the microstructure, the auxetic effect in these materials can be more or less extreme. Most auxetic foams expand by a factor of about 30 percent or so before shredding because of the stretching force. With more advanced auxetics structured on the molecular level, more impressive expansion might be possible.
Proposals for the use of auxetics have been fairly wide in scope, although few implementations had actually been created as of 2011. Auxetics used in small medical probes could be used to dilate blood vessels. These materials expand so readily that they also would be ideal filters, capable of catching many foreign particles in their macrostructure. Unlike traditional filters, they could remain small and compact when not in use.
Threading auxetic fibers through composites could allow for strength improvements, with the tendency to expand under stretching stress helping keep the overall structure of the composite together. This is particularly true of composites consisting of materials that have a tendency to slide past each other. Many other potential applications for auxetics are yet to be developed, although the list is long and shows great promise in many fields.